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Plutonic xenoliths from St Vincent, Lesser Antilles, act as ‘windows into the deep’

Coarse-grained igneous rocks, sourced directly from a sub-volcanic magma chamber, provide glimpses into the prevailing conditions beneath an active volcano.

Tollan and co-workers used several different techniques to analyse the major and isotopic composition of mineral phases in cumulates sourced from the active volcanic island of St Vincent in the Lesser Antilles. Cumulates are igneous rocks comprise the first fractionating minerals that form from a crystallising melt. The combinations of minerals and their composition are modulated by the conditions imposed upon the magma when it is cooling, and thus each rock represents a unique ‘snapshots’ of magmatic evolution.

The study revealed that the rounded cumulate nodules are distinctively rich in anorthitic plagioclase and pargasitic hornblende, accompanied by fresh olivine and pyroxenes. The composition of the minerals indicates the cumulates formed at ~970 – 1150°C at 5-6 km below the Earth’s surface.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

The paper concludes that all the cumulates collected from St Vincent formed from relatively evolved melts rich in calcium, aluminium and water. These types of magmas are produced from early crystallisation of mafic phases (olivine, clinopyroxene and Cr-rich spinel) before their low density facilitates ascent through the crust, where they stall and deposit the observed cumulus minerals in shallow magma chambers. Evidence from oxygen isotopes suggests the cumulates have a residence time of ~50,000 years before being entrained and fragmented by newly injected magmas, and transported to the surface during explosive volcanic eruptions.

Full reference: Tollan, PME, Bindeman, I & Blundy, JD (2012) ‘Oxygen and hydrogen isotope compositions of plutonic xenoliths from St. Vincent, Lesser Antilles Island Arc.’ Contributions to Minerology and Petrology, no. 163, pp. 189-208. http://dx.doi.org/10.1007/s00410-011-0665-9

SCIENTIFIC ABSTRACT

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine-gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4-10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89-5.18‰), plagioclase (5.84-6.28‰), clinopyroxene (5.17-5.47‰) and hornblende (5.48-5.61‰) and hydrogen isotope composition of hornblende (δD = -35.5 to -49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.

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